The present invention relates to a method of manufacturing a semiconductor device. More particularly, the invention relates to a method of forming a stable and dense insulating film of silicon nitride on a silicon surface by a thermal nitriding reaction.
Conventionally, in the manufacture of a semiconductor device, the area on the semiconductor surface in which the junction is exposed is covered with an insulating film in order to suppress the aging variation of the electrical characteristic and to obtain high reliability.
Furthermore, in a field effect transistor, or FET, having a metal insulating film semiconductor, or MIS, structure and an integrated circuit, or IC, a gate insulating film is provided for inducing the inversion layer channel. Stability of the characteristic is therefore particularly required.
Generally, silicon, or Si, has been utilized as the semiconductor substrate, silicon dioxide, or SiO.sub.2, has been utilized as the insulating film material and aluminum, or Al, has been utilized as the metal. However, such a structure has the disadvantage that the gate threshold voltage varies considerably when an electric field as high as 10.sup.6 V/cm is applied to the insulating film at a temperature of about 200.degree. C. This phenomenon appears to occur as a result of a variation of the space charge distribution at the interface, because there are the trapping levels of the carrier due to structural defects of the SiO.sub.2 and a drift of impurity ions migrating into the silicon dioxide film. It is obvious that the effect of such variation of the space charge due to the structural defect at the interface of the silicon substrate and the insulating film becomes most distinctive. For this reason, various coating techniques such as, for example, the vapor growth method and sputtering, etc., have been investigated, instead of silicon dioxide film, by thermal oxidation. Thermal oxidation easily permits the mingling of ions such as alkali ions but cannot reduce the defect at the interface more than that of the silicon dioxide film.
On the other hand, a silicon dioxide film produced by thermal oxidation in a very clean ambient maintains a defect of 10.sup.11 /cm.sup.2 in terms of the surface charge density, but a defect such as excessive silicon ions etc., remains at the interface. This is a serious problem. Thus, in order to solve the problem of the insulating film formed on the semiconductor surface, a method is proposed for forming a homogeneous amorphous silicon nitride film on the silicon by substantially the thermal nitriding method.
It is a known fact that the nitriding proceeds when the silicon substrate is heated in a nitrogen or ammonia ambient. In accordance with prior experimentation, however, it is considered difficult to obtain homogeneous amorphous silicon nitride, or Si.sub.3 N.sub.4. The present inventors have discovered the cause to be that when a natural oxide film or impurity particle is adhered to the silicon, the silicon nitride is crystallized, due to such substances considered as a nucleus, and grows, or local nitriding proceeds non-homogeneously.
The principal object of the invention is to provide a method of forming a homogeneous insulating film capable of blocking ionic impurities and having few defects at the interface on the surface of a silicon substrate.
An object of the invention is to provide a thermal nitriding method for producing a homogeneous amorphous silicon nitride film.
Another object of the invention is to provide a thermal nitriding method which includes a method for effectively eliminating a natural oxide film and impurity particles on the surface of a silicon substrate and thereby forming a homogeneous silicon nitride film.